Mass spectrometry method and apparatus

ABSTRACT

A mass spectrometer  10  comprises an ion source  12  which generates nebulized ions which enter an ion cooler  20  via an ion source block  16 . Ions within a window of m/z of interest are extracted via a quadrupole mass filter  24  and passed to a linear trap  30 . Ions are trapped in a potential well in the linear trap  30  and are bunched at the bottom of the potential well adjacent an exit segment  50 . Ions are gated out of the linear trap  30  into an electrostatic ion trap  130  and are detected by a secondary electron multiplier  10 . By bunching the ions in the linear trap  30  prior to ejection, and by focussing the ions in time of flight (TOF) upon the entrance of the electrostatic trap  130 , the ions arrive at the electrostatic trap  130  as a convolution of short, energetic packets of similar m/z. Such packets are particularly suited to an electrostatic trap because the FWHM of each packet&#39;s TOF distribution is less than the period of oscillation of those ions in the electrostatic trap.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 10/472,917, filed Sep. 23, 2003, entitled “Mass SpectrometryMethod and Apparatus,” which is a national stage application under 35U.S.C. §371 of PCT Application No. PCT/GBO2/01373, filed Mar. 20, 2002,entitled “Mass Spectrometry Method and Apparatus,” which claims thepriority benefit of United Kingdom Patent Application Nos. 0107280.8,filed Mar. 23, 2001 and 0126764.0, filed Nov. 7, 2001, whichapplications are incorporated herein by reference in their entireties.

FIELD OF INVENTION

This invention relates to a method and an apparatus of massspectrometry, and in particular to a method and an apparatus for storageand injection of ions into an electrostatic ion trap.

BACKGROUND OF THE INVENTION

Mass spectrometers have been used to analyse a wide range of materials,including organic substances such as pharmaceutical compounds,environmental compounds and biomolecules. They are particularly useful,for example, for DNA and protein sequencing. In such applications, thereis an ever increasing desire for high mass accuracy, as well as highresolution of analysis of sample ions by the mass spectrometer,notwithstanding the short time frame of modem separation techniques suchas gas chromatography/mass spectrometry (GC/MS), liquidchromatography/mass spectrometry (LC/MS) and so forth.

One of the new directions in the field of mass spectrometry is thedevelopment of mass analysers where ions are dynamically trapped in anelectrostatic field. Broadly, these may be divided into two classes:those that employ frequency analysis by image current detection, asdisclosed in U.S. Pat. No. 5,880,466 and U.S. Pat. No. 5,886,346, andthose that employ time of flight (TOF) separation and ion detection bysecondary electron conversion, as is disclosed, for example by H.Wollnik, in J. Mass Spectrom. Ion Proc. (1994), vol. 131, at pages387-407, and by C. Piadyasa et al., in Rapid Commun. Mass Spectrom.(1999), vol. 13 at pages 620-624. Although the trap fields may be rampedat the beginning of the mass scan, they are typically held very stableduring the detection, or TOF separation of ions, and so each of theforegoing mass analysers may be regarded as electrostatic traps (ESTs).

Such EST mass analysers can achieve high and even ultra-high massresolutions (in excess of 100,000), thus allowing more accuratedetermination of ion masses. However, they all operate using aninherently pulsed technique and as such the task of coupling to anyexternal continuous ion source is a serious problem.

To improve duty cycle and sensitivity, it is possible to use an externalcollision quadrupole ion trap for ion cooling and storage betweeninjections. This technique has proved particularly successful whencombined with other inherently pulsed techniques such as TOF massanalysis as is described by S. Michael et al., in Rev. Sci. Instrum.(1992) vol. 63, pages 4277 to 4284. Here, ions are accumulated in thetrap. As suggested in U.S. Pat. No. 5,572,022, it is possible to controlthe number of ions in the trap to reduce space-charge effects. Once ionshave been stored in the trap, they can be pulsed into the TOF massanalyser by applying high voltages to the (normally grounded) end capsof the trap. In U.S. Pat. No. 5,569,917, the ions are given asimultaneous “push” out of the trap and a “suck” from the TOF massanalyser, so as to improve the efficiency of ion injection into theanalyser. The spatially spread ion beam is focussed into a tight pack inthe “object” plane of the TOF mass analyser.

Despite these improvements, quadrupole ion traps are still currently arelatively inefficient technique for injecting ions into a mass analyser(down to a few percent), and they also suffer from low space chargecapacity due to the limited trap volume.

One approach that has been taken to address these problems is to employa different type of collisonal storage device known as a linear trap(LT) or RF multipole trap. U.S. Pat. No. 5,179,278 shows such anarrangement, wherein a two-dimensional multipole RF field is generated.The trap of U.S. Pat. No. 5,179,278 is limited by end lenses.Alternatively, the poles of the trap may be split into sections as isshown in U.S. Pat. No. 5,420,425. Both split poles and end caps can beemployed together. The elevated voltages on the end lenses or sectionslimit the ion movement along the axis whilst the RF voltage provides aquasi-potential well in the radial direction. If ions lose enough energyduring the first passes through the multipole, then they may be trappedin it and squeezed towards the axis during further collisions. Thenumber of ions in the trap can be controlled using a short pre-scan, atechnique disclosed in the above-referenced U.S. Pat. No. 5,572,022.Nevertheless, to inject ions from the LT into the next stage ofanalysis, the voltage is lowered on the exit lens and the ions in the LTare allowed to flow out of the multipole. This flow typically lasts upto hundreds or even thousands of microseconds. These time scales arecompatible with the injection times for quadrupole ion traps (asdisclosed in the above-referenced U.S. Pat. No. 5,179,278) or forFourier Transform Ion Cyclotron Resonance (FTICR) as set out by M. Senkoet al in JASMS, (1997), volume 8, pages 970-976. The time scales arealso suitable for orthogonal acceleration TOF mass spectrometry, see forexample U.S. Pat. No. 6,011,259, U.S. Pat. No. 6,020,586, andWO99/30350.

Segmented construction of the poles in the LT may be employed, as setout by M. Belov et al, in Analytical Chemistry (2001) volume 73, pages253-261, to reduce the injection time down to about 300-400microseconds. The segmented construction of the LT provides an axialfield which causes ions to be displaced towards the exit lens.

Even so, such injection times are too long for an electrostatic trap.This is because ESTs require high ion energies (typically 1-2 keV percharge) to achieve dynamic trapping. If injection takes place overhundreds of microseconds, at such energies the process may last forhundreds of ion reflections. Without any collisional cooling inside theelectrostatic trap, ion stability may be compromised.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method and anapparatus which alleviates these problems with the prior art. Inparticular, it is an object to provide a method and an apparatus whichprovides for adequate storage of ions prior to injection of these ionsinto an electrostatic trap over a timescale compatible with such adevice.

According to a first aspect of the present invention, there is provideda method of injection of sample ions into an electrostatic trap,comprising the steps of: (a) generating a plurality of sample ions to beanalysed, each of which has a mass-to-charge ratio m/z; (b) receivingthe sample ions through a storage device entrance in an ion storagedevice having a plurality of storage device poles; (c) supplying atrapping voltage to the storage device so as to trap at least aproportion of the received sample ions within a volume ρ in the storagedevice, during at least a part of a trapping period, the thus trappedions each having a kinetic energy E_(k) such that there is an averagekinetic energy {overscore (E)}_(k) of the ions in the volume ρ duringthe trapping period or the said part thereof; (d) supplying a releasevoltage to the storage device so as to controllably release at leastsome of the said sample ions contained within the said volume of thestorage device from a storage device exit, the release voltage being ofa magnitude such that the potential difference then experienced by theions across the volume ρ is greater than the said average kinetic energy{overscore (E)}_(k) during the trapping period or said part thereof, andfurther wherein the release voltage is such that the strength of theelectric field generated thereby at any first point across the volume ρ,upon application of the said release voltage, is no more than 50%greater or smaller than the strength of the electric field generatedthereby at any other second point across the volume ρ; (e) receivingthose sample ions released from the storage device exit according to thecriteria of step (d) through an entrance of an electrostatic trap havinga plurality of trapping electrodes, the ions arriving as a convolutionof bunched time of flight distributions for each m/z, each distributionhaving a full width at half maximum (FWHM); and (f) trapping thereceived sample ions within the electrostatic trap by applying apotential to the electrodes such that the sample ions describe movementhaving periodic oscillations in at least one direction.

The method of the present invention proposes particular restrictions onthe release potential supplied to the storage device which ensures thatgroups of ions of a given m/z arrive as a tightly focussed “bunch” at oradjacent the electrostatic trap. The two conditions are, respectively,that the electric field experienced by the ions upon leaving the storagedevice is relatively uniform and that the potential drop experienced bythe ions upon release is larger than the average thermal (kinetic)energy of the ions when trapped in the storage device, and preferablymuch larger.

The first condition is a consequence of the effective focussing of ionsin time of flight from the storage device and into the electrostatictrap. The focal length, L, is given by L=αV/E, where V is the finalenergy of the ions when released from the storage device, E is theelectric field strength, and a is a constant. If the electric field isnon-uniform, therefore, along the volume ρ, then ions of the same m/zwill be focussed at different lengths L and will not arrive in the sametightly focussed bunch. A variation in the electric field strength of nomore than about 30%-50% is preferred.

The second criterion ensures that the thermal energy (i.e., the kineticenergy) of the trapped ions is less than and preferably insignificantrelative to the gradient of the potential ‘slope’ upon which the ionsfind themselves when the release potential is applied.

In an idealized storage device or ion trap, the ions are all in the sameplace and leave with exactly the same energy when ejected. This meansthat they arrive at the same time at any chosen location downstream ofthe ion trap. In reality, of course, the ions have a range of kineticenergies and start off from different locations within the trap. Hencethe ions of the same m/z arrive at different times downstream of the iontrap. The purpose of TOF focussing is therefore to cause ions furtherback in the trap to ‘catch up’ with ions ejected from the front of thetrap, by ensuring that the ions nearer the front of the trap move moreslowly than those leaving later. The release potential is chosen so thatthe ions are ejected without being affected significantly by the randomperturbations of thermal energy spread. Although it is necessary thatthe potential drop is at least as much as the average kinetic energy, amultiple of two is preferred, of five is more preferable, and at leastone order of magnitude is most preferable.

This second condition provides the further advantage that the ions willbe relatively energetic upon arrival at or adjacent the electrostatictrap. Electrostatic traps have an energy acceptance window, which iscentred at a relatively high energy, so that the ions ejected inaccordance with the conditions of the present invention are within thatenergy acceptance window.

Provided that the foregoing conditions are met, the ions will arrive atthe electrostatic trap as a convolution of short, energetic packets ofsimilar m/z. Such packets are ideally suited to an electrostatic trap(and particularly the preferred embodiment of an orbitrap) because theFWHM of each ion packet's TOF distribution for a given m/z ratio is thenless than the period of oscillation of those sample ions having that m/zwhen in the electrostatic trap. In other words, the packets aresufficiently coherent for detection to take place.

In preferred embodiments, the ions are pre-cooled, for example in thestorage device. This reduces the thermal energy of the ions and alsotheir energy spread, hence reducing the ratio of kinetic energy torelease voltage which is the second prerequisite set out above.Pre-cooling may be achieved by collisional cooling, for example.Furthermore, the trapping voltage may be applied so as to force the ionsin the storage device towards the exit thereof. This may either becarried out throughout the trapping period or may, in preference, becarried out only immediately prior to ejection.

To obtain bunching of the sample ions at the electrostatic trap, it ispreferable to employ an axially segmented linear trap (LT) as thestorage device. A differential, preferably DC, voltage is appliedbetween the two or more segments of the LT so as to force the ion cloudwithin the LT towards the exit thereof. This procedure may be carriedout after trapping and cooling of the sample ions received from an ionsource in the storage device, which may be achieved by applying voltagesto the pole segments so as to create an axial potential well whose baseis in the middle of the LT. Alternatively, the bottom of the potentialwell may be located from the outset at or towards the exit from thestorage device. It is particularly preferable that the bottom of thepotential well is no more than twice the diameter inscribed by the polesof the storage device away from the storage device exit.

The ions in the storage device are preferably gated out of the exitthereof by applying one or more voltage pulses, to an end electrode ofthe storage device for example.

The condition for correct focussing or bunching of the sample ions at oradjacent the electrostatic trap is met, in preferred embodiments, by therequirement that the period of oscillation of those ions when injectedinto the electrostatic trap is shorter, and preferably much shorter,than the time of flight of those sample ions between the storage deviceexit and arrival at the electrostatic trap entrance.

In alternative embodiments, the further condition that the FWHM time offlight distribution of the ions arriving at the electrostatic trapshould be less than the TOF along each detection electrode in theelectrostatic trap is imposed as well.

In a particularly preferred embodiment, the storage device is arrangedto receive ions along a first direction and to release them from thestorage device along a second, orthogonal direction. This permits muchhigher space charge capacity and better ion beam parameters.

In that case, it is preferable that the storage device should be curvedalong the first direction. This improves geometric focussing.Optionally, lenses may be added to convert a wide angle beam into anarrow beam.

The method may also include boosting the ion energies prior to theirarrival at the electrostatic trap entrance.

In preferred embodiments, the method is employed for the generation,storage and detection of sample ions in MS-only mode. Alternatively, themethod may be employed for collision-induced dissociation of the ions toproduce daughter sample ion fragments. In either case, it is preferableto select the release voltage so as to focus the ions on the entrance tothe orbitrap. In an alternative embodiment, however, the method may beemployed to detect fragment ions using surface-induced dissociation. Inthis case, the method preferably further comprises focussing the sampleions through the electrostatic trap and onto a collision surface. Thefragment ions which result are then accelerated back towards theelectrostatic trap.

It is particularly preferable that the ions arrive at the electrostatictrap at an angle tangential to a central plane of the electrostatictrap. This is preferably achieved by lenses between the ion trap and theelectrostatic trap. The benefit of this is that no further excitation ofthe ions is necessary once they enter the electrostatic trap. This inturn reduces the amount of electronics necessary for correct operationof the electrostatic trap, and is to be compared with the arrangement ofU.S. Pat. No. 5,886,346 referenced above.

The lenses between the ion trap and electrostatic trap, where present,preferably offer no direct line of sight between the inside of the iontrap and the inside of the electrostatic trap. This arrangement preventsstreaming of ions and gas carryover from the (relatively high pressure)ion trap into the (relatively lower pressure) electrostatic trap.

Detection of the sample ions in the electrostatic trap may be achievedin a number of ways. Most preferably, the electrostatic trap is of theorbitrap type and ions are trapped in a hyper-logarithmic field. Asbunches of coherent ions of different m/z pass by the outer electrodesof the orbitrap, an image current is induced therein. This current maybe amplified and then processed to generate a TOF spectrum, for exampleby Fourier transform analysis.

The field within the electrostatic trap may preferably be compensated byapplying a compensating voltage (which may be time dependent) to a fieldcompensator during detection of the ions. This procedure ensures minimumfield perturbation within the volume occupied by the ion trajectories.Additionally or alternatively, during ion injection into theelectrostatic trap, the field compensator may be arranged to act as adeflector to improve the trapping efficiency of the electrostatic trip.

As with previous ion traps, and LTs in particular, the ion trap maycontain facilities for resonance or mass-selective instability scans toprovide for data-dependent excitation, fragmentation or elimination ofcertain m/z ratios.

The optimum duration of ion trapping in the ion trap may be determinedprior to commencement of mass analysis by carrying out a pre-scan.Preferably, a secondary electron multiplier (SEM) or the like isemployed. The SEM may be located radially of the ion trap and in thatcase mass-selective instability or a resonance excitation scan in theion trap may be used. Most preferably, however, an axial SEM is employeddownstream of the electrostatic trap and on an ion beam axis. In thiscase, ions are preferably injected into the electrostatic trap just asthey would be for subsequent mass analysis.

According to a second aspect of the present invention, there is provideda mass spectrometer comprising: (a) an ion source arranged to supply aplurality of sample ions to be analysed, each of which has amass-to-charge ratio m/z; (b) an ion storage device comprising aplurality of storage device poles and having a storage device entranceend through which the said sample ions are received and a storage deviceexit end through which the said sample ions may exit; (c) a voltagesource arranged to supply a trapping voltage to the storage device polesso as to contain at least a proportion of the sample ions receivedthrough the storage device entrance end of the storage device within avolume ρ of the storage device in a trapping mode during at least a partof a trapping period, the thus trapped ions each having a kinetic energyE_(k) such that there is an average kinetic energy {overscore (E)}_(k)of the ions in the volume ρ during the trapping period or the said partthereof, and to supply a release voltage to the storage device in an ionejection mode so as to controllably release at least some of the saidsample ions contained within the said volume ρ of the storage devicethrough the storage device exit end, the release voltage being of amagnitude such that the potential difference then experienced by theions across the volume ρ is greater than the said average kinetic energy{overscore (E)}_(k) during the trapping period or said part thereof, andfurther wherein the release voltage is such that the electric fieldgenerated thereby at any first point across the volume ρ, uponapplication of the said release voltage, is no more than 50% greater orsmaller than the electric field generated thereby at any other secondpoint across the volume ρ; and (d) an electrostatic trap having anelectrostatic trap entrance arranged to receive those ions releasedthrough the storage device exit end and meeting the criteria imposed bythe applied trapping and release potentials, as a convolution of bunchedtime of flight distributions for each m/z, each distribution having afull width at half maximum (FWHM); the electrostatic trap furthercomprising a plurality of electrodes arranged to trap ions receivedthrough the electrostatic trap entrance therebetween so that the saidtrapped ions describe movement having periodic oscillations in at leastone direction.

As with the method of the first aspect, a segmented multipolar ion trapis preferable. In that case, to maximise focussing or bunching of ionpackets at the entrance to the electrostatic trap, the length of thesegment of the pole pieces proximal the ion trap is preferably shorterthan twice the inscribed diameter between the segments of the ion trap(measured radially), and most preferably shorter than the inscribeddiameter. Also, the distance between the ion trap exit and the centre ofthe segment closest thereto is preferably greater than or equal to thesaid inscribed diameter.

Other features and advantages of the invention will become apparent withreference to the appended claims and to the following specificdescription.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be put into practice in a number of ways, andsome specific embodiments will now be described by way of example onlyand with reference to the accompanying drawings in which:

FIG. 1 is a schematic side view of a mass spectrometer embodying thepresent invention and including an ion trap and electrostatic trap;

FIG. 2 shows schematically and in perspective a part of the ion trap ofFIG. 1;

FIG. 3 shows a front view of the electrostatic trap of FIG. 1;

FIG. 4 a shows, schematically, the potential distribution in the iontrap of FIGS. 1, 2 and 3 when ions are trapped therein;

FIG. 4 b shows, schematically, the potential distribution in the iontrap of FIGS. 1, 2 and 3 at the point when trapped ions are ejected fromthe ion trap;

FIG. 5 shows a front view of an alternative electrostatic trap for usein the mass spectrometer of FIG. 1;

FIG. 6 shows an alternative configuration of the mass spectrometer ofFIG. 1, again in schematic side view and including a curved ion trapalong with an electrostatic trap;

FIG. 7 shows a sectional view through the curved ion trap of FIG. 6; and

FIG. 8 shows a side view of an electrostatic trap for use with the massspectrometer of FIG. 1 OR FIG. 6, in combination with a collisionsurface downstream of the electrostatic trap to allow operation of themass spectrometer in surface-induced dissociation mode.

DETAILED DESCRIPTION

Referring first to FIG. 1, a mass spectrometer 10 is shown. The massspectrometer comprises a continuous or pulsed ion source 12, such as anelectron impact source, an electro-spray source (with or without acollisional RF multipole), a matrix-assisted laser desorption andionisation (MALD) source, again with or without a collisional RFmultipole, and so forth. In FIG. 1, an electrospray ion source 12 isshown.

The nebulized ions from the ion source 12 enter an ion source block 16having an entrance cone 14 and an exit cone 18. As is described, forexample, in WO 98/49710, the exit cone 18 has an entrance at 90° to theion flow in the block 16 so that it acts as a skimmer to preventstreaming of ions into the subsequent mass analysis components.

A first component downstream of the exit cone 18 is an ion cooler 20which reduces the energy of the sample ions from the ion source 12.Cooled ions exit the ion cooler 20 through an aperture 22 and arrive ata quadrupole mass filter 24 which is supplied with a D.C. voltage uponwhich is superimposed an arbitrary r.f. signal. This mass filterextracts only those ions within a window of m/z of interest and thechosen ions are then released to a linear ion trap 30. The ion trap 30is segmented, in the embodiment of FIG. 1, into an entrance segment 40and an exit segment 50. Although only two segments are shown in FIG. 1,it will be understood that three or more segments could instead beemployed. As better seen in FIG. 2, the segments 40, 50 are each formedfrom four rods which are radially spaced so as to form a trapping volume60 between them.

To trap ions within the trapping volume 60, a voltage source (not shown)applies an RF voltage to each of the segments 40, 50. The application ofan RF field generates a potential well in the axial direction.Collisions between ions entering the linear trap 30 rapidly cause theseions to sink towards the bottom of the potential well.

The ends of the linear trap 30 are bounded by exit and entranceelectrodes 70, 80 respectively. These electrodes are supplied with a DCvoltage V_(D) and V_(a) respectively. As will be familiar to thoseskilled in the art, the linear trap 30 may also contain facilities forresonance or mass-selective instability scans, to provide data-dependentexcitation, fragmentation or elimination of selected mass-to-chargeratios.

In preference, the length of the exit segments 50 is not in excess ofthe inscribed diameter D between the rods (FIG. 2). Also, the distance xbetween the exit electrode 70 and the axial centre of the exit segment50 is preferably comparable to, or greater than, the inscribed diameterD (again, see FIG. 2).

The linear trap 30 may have a pressure gradient therein. In this way,the conditions in one part of the trap 30 are optimised for the bestdissipation of energy through ion collisions, whilst near the exitelectrode 70, the conditions may be optimised for the best trapping,lowest fragmentation and so forth. The pressure gradient may, forexample, be created through the introduction of additional gas inlets.

Downstream of the exit electrode is a deflection lens arrangement 90including deflectors 100, 110. The deflection lens arrangement isarranged to deflect the ions exiting the linear trap 30 in such a waythat there is no direct line of sight connecting the interior of thelinear trap 30 with the interior of an electrostatic orbitrap 130downstream of the deflection lens arrangement 90. This preventsstreaming of energetic ions from the relatively high pressure lineartrap 30 into the relatively low pressure orbitrap 130. The deflectionlens arrangement 90 also acts as a differential pumping aperture.

Downstream of the deflection lens arrangement 90 is a conductivityrestrictor 120. This sustains a pressure differential between theorbitrap 130 and the lens arrangement 90.

Ions exiting the deflection lens arrangement 90 through the conductivityrestrictor 120 arrive at the orbitrap 130. The orbitrap 130 has acentral electrode 140 as may better be seen with reference now to FIG.3. The central electrode 140 is connected to a high voltage amplifier150.

The orbitrap 130 also contains an outer electrode split into two outerelectrode parts 160, 170. Each of the two outer electrode parts 160, 170is connected to a differential amplifier 180. Preferably, thisdifferential amplifier is maintained at virtual ground.

Referring once more to FIG. 1, downstream of the orbitrap 130 is asecondary electron multiplier 190, on the optical axis of the ion beam.Although not shown in FIG. 1, the secondary electron multiplier (SEM)190 may also be located on the side of the linear trap 130.

The system, and particularly the voltages applied to the various partsof the system, is controlled by a data acquisition system. This dataacquisition system is in itself known and does not form a part of thepresent invention. Accordingly, it is not shown in the Figures and willnot be described further. The data acquisition system may also carry outsignal processing as described below. Likewise, a vacuum envelope isalso provided, to allow differential pumping of the system. Again, thisis not shown in the Figures, although the typical pressures areindicated in FIG. 1.

In operation, ions from the ion source 12 enter the segmented lineartrap 30 and are reflected by an elevated potential VD on the exitelectrode 70 thereof. AC voltages at RF frequencies are applied to thesegments of the trap to provide a quasi-potential well in the radialdirection whilst DC voltages V_(a), V_(b) and V_(c) provide a potentialwell along the axis of the linear trap 30. The pressure inside thelinear trap 30 is chosen in such a way that ions lose sufficient kineticenergy during their first pass through the trap that they accumulatenear the bottom of the axial potential well. Before ions are removedfrom the linear trap 30, the DC voltages V_(a), V_(b), V_(d) and V_(D)may be varied in such a way that the centre of ion cloud within thelinear trap 30 is shifted into the end section of the linear trap, thatis, into the volume defined between the rods in the exit segment 50adjacent to the exit electrode 70. As an alternative, the bottom of theaxial potential well may instead be located in this exit segment 50 fromthe start of ion storage in the linear trap 30.

FIG. 4 a shows, not to scale, the potential electrode 80 and the exitelectrode 70 when ions are trapped therein. It will be seen that theions sit in a potential well defined by the difference in potentialsbetween the exit electrode 70, the exit segment 50 of the trap 30 andthe entrance segment 40 thereof.

At the end of storage, the data acquisition system starts to ramp thevoltage applied to the central electrode 140 in the orbitrap 130 and,simultaneously, applies a voltage pulse to the exit electrode 70 of thelinear trap 30. In presently preferred embodiments, a single pulse isapplied to empty the linear trap. However, multiple pulses may beemployed instead. In any event, the delay between successive pulses ischosen in such a way that all of the mass range of interest arrives intothe orbitrap 130 during the correct phase of the voltage applied to thecentral electrode 140 thereof as it is ramped. Although the ramping ofthe voltages on the central electrode of the orbitrap 130 and the exitelectrode 70 of the linear trap are timed to each other, they do nothowever need to be synchronous. Thus, the voltage applied to the centralelectrode 140 of the orbitrap 130 may start to ramp before the pulse isapplied to the exit electrode 70 on the linear trap, and may continue toramp for a period (e.g. tens of microseconds) after the linear trap hasbeen emptied.

FIG. 4 b shows the potential at the different points along the ion trap30 between the entrance 80 and the exit electrode 70 again not to scale,when such a pulse is applied to the exit electrode 70. Because the pulseis negative (in the convention adapted to illustrate the specificembodiment described, the ions previously trapped in the potential wellinstantaneously find themselves on a “slope” which accelerates them awayfrom the ion trap 30. As will be explained below, the ejection techniquecauses the ions leaving the ion trap 30 to be time-of-flight focussedonto the entrance of the electrostatic trap 130.

It is important, for correct trapping of ions in the orbitrap 130, thatthey arrive at the entrance to the orbitrap when the voltage on thecentral electrode 140 thereof is between approximately (D₁/D₂)^(1/2) V,and V, where V is the final, static voltage on the central electrode140, D₁ is the outer diameter of the central electrode 140, and D₂ isthe inner diameter of the outer electrode formed from the outerelectrode parts 160 and 170. The ion energy at the entrance to theorbitrap 130 also needs to lie within a certain range.

Whilst the voltage applied to the central electrode 140 of the orbitrap130 is ramped, ions are directed and focussed by the linear trap 30 andthe deflection lens arrangement 90 to the entrance of the orbitrap 130.The ions enter the field within the orbitrap 130 tangentially to theouter electrodes formed from the outer electrode parts 160, 170 and areprevented from hitting this electrode again by amonotonically-increasing electric field, which squeezes the ions closerto the centre of the trap. Tangential injection into the orbitrap 130 isachieved by displacing the trap relative to the centre of the beam ofions arriving from the ion trap 30. By way of example only, the orbitrap130 may be positioned so that ions enter it at a radius of 17.4 mm withz=10 mm, the highest internal radius of the electrodes being 20 mm inthis specific example.

The rise time of the electric field depends on the mass range to betrapped, ion parameters, and the orbitrap 130, but is usually between 20and 200 microseconds. Squeezing stops when there is no more threat oflosing ions onto the electrodes.

The orbitrap 130 is shaped so as to generate a hyper-logarithmic fieldbetween the central electrode 140 and the outer electrode formed fromthe outer electrode parts 160, 170. The potential distribution of thishyper-logarithmic field may be described in cylindrical coordinates (r,z) by the following equation:${V\left( {r,z} \right)} = {{\frac{k}{2}\left\lbrack {z^{2} - \frac{r^{2}}{2}} \right\rbrack} + {\frac{k}{2}\left( R_{m} \right)^{2}{\ln\left\lbrack \frac{r}{R_{m}} \right\rbrack}} + C}$

where z=0 is the plane of symmetry of the field, C, k, R_(m)(>0) areconstants, and k>0 for positive ions. Such a field creates a potentialwell along the z axis direction which causes ion trapping in thatpotential well provided that the incident energy is not too great forthe ion to escape. As the voltage applied to the centre of electrode 140increases, the field intensity increases and therefore the force on theions towards the longitudinal axis increases, thus decreasing the radiusof spiral of the ions as may be seen from FIGS. 1 and 2. Thus, the ionsare forced to rotate in spirals of smaller radius as the sides of thepotential well increase in gradient.

There are three characteristic frequencies of oscillation within thehyper-logarithmic field. The first is the harmonic motion of the ions inthe axial direction where they oscillate in the potential well with afrequency independent of energy in this direction. The secondcharacteristic frequency is oscillation in the radial direction sincenot all of the trajectories will be perfectly circular. The thirdfrequency characteristic of the trapped ions is the frequency of angularrotation.

Further details of the preferred electrode arrangement of the orbitrap130 may be found in U.S. Pat. No. 5,886,346, referenced above, thecontents of which are incorporated by reference in their entirety. Itwill, however, be noted that, in the present case, the ions enter thefield tangentially and do not require a separate injection of radialforce which in turn reduces the amount of electronic control of theorbitrap 130 that is necessary.

The ion packets arriving at the entrance to the orbitrap 130 are bunchedtogether due to the time of flight focussing created by the ion ejectiontechnique from the linear trap 130. The ion packets are sufficientlycoherent that coherent axial oscillation within the orbitrap 130 takesplace without addition excitation. The required degree of coherencydepends on the type of detection.

If all ions of the same mass-to-charge ratio were to have the sameinitial kinetic energy, begin flight at the same time, and from the sameposition within the trap, then they would all leave the trap togetherand travel together to arrive at exactly the same moment at any pointdownstream of the trap. This idealized situation cannot of course berealized in practice, primarily due to three factors which ‘smear’ theinitial peak from a delta function. Firstly, the starting position ofdifferent ions of the same mass-to-charge ratio will be different,secondly, the time at which flight begins, and thirdly the initialkinetic energies of the different ions of the same mass-to-charge ratiowill be different.

The present invention addresses the non-ideal nature of the ions in thetrap by firstly minimizing the spread of initial kinetic energies and by‘bunching’ the ions together at one end of the trap (so that they tendto leave from roughly the same point in the trap) and secondly byfocussing the ions during flight so that any temporal, spatial orenergetic spread which still remains is reduced. FIG. 4 b shows why thisshould be so.

When the exit electrode 70 receives the negative going pulse to ejectthe ions, those ions of the same m/z, which at the moment of pulsing areclosest to the exit electrode 70, experience a smaller voltage drop thanthose further away from the exit electrode 70. As the ions of similarm/z which are closest to the exit aperture have a smaller distance totravel to that exit aperture, they pass through it earlier than the ions‘behind’ them but at a lower velocity. In other words, ions further awayfrom the exit aperture 70 on application of the negative pulse takelonger to be emptied from the tap but leave it at a higher velocity. Inthis manner, the ions bunch up downstream of the ion trap 30. Bycarefully selecting the ejection parameters, the ions in the ion trap 30may be focussed onto the entrance of the orbitrap 130.

In the preferred embodiment shown in FIGS. 1 and 3, a mass spectrum isgenerated using image current detection, which technique is againdescribed in U.S. Pat. No. 5,886,346. An interference pattern offrequencies of different mass-to-charge ratios produces an image currenton the outer electrode parts 160, 170. This current is amplified by thedifferential amplifier 180 and then processed by the data acquisitionsystem by application of a Fourier transform. For this type of iondetection, coherency of the ion packets is achieved as soon as theduration dT (m/z) of a given ion packet of a specific m/z is smallerthan the period of axial oscillations within the orbitrap 130. Thisperiod of axial oscillation may in turn be significantly less than thetime of flight between the linear trap 30 and the entrance to theorbitrap 130. To achieve this level of coherence, time of flightfocussing or bunching of the ions as they enter the orbitrap 130 isnecessary. It is important to distinguish this bunching from first orsecond order focussing typical in time of flight (TOF) mass analysers.The time of flight between the linear trap 30 and the orbitrap 130 isnormally significantly greater than the period of axial oscillationswithin the orbit trap 130 because of the differential pumping betweenthe linear trap 30 and the orbitrap 130 which necessarily requires asignificant spatial separation between the linear trap and the orbitrap130.

The electric field strength and the ion trap dimensions are chosen insuch a way that the ions catch up, that is, focus in time of flight,just at the entrance to the EST 130. For example, for a uniform field,the focal point is located 3.d away from the starting point where d isthe length of this uniform field as measured from the starting point tothe field boundary. Non-uniform fields have focussing properties but themathematical relations are very complex. The random energy distributionsof the ions trapped in the ion trap 30 adversely affect the quality ofthe TOF focussing; for example, the pulse width becomes comparable withthe total time of flight as soon as the energy spread of the ionsbecomes comparable with the acceleration voltage. In order to addressthese problems, therefor, the bunching of the ions is achieved, inpractice, through a combination of the following requirements.

Firstly, the relative variation of the electric field strength along theion cloud within the ion trap 30, or at least the portion to be injectedinto the orbitrap 130, should be smaller than unity, and preferably muchsmaller than unity (such as 10% or even less). Such uniformity of thefield along the beam may be achieved by ion squeezing prior to pulsingof the linear trap 30 which in turn is achieved by moving the base ofthe potential well in the linear trap 30 towards the exit electrode 70as described above. Collisional cooling of ions within the linear trapalso assists in this squeezing. Secondly, the voltage drop across theion cloud in the linear trap should at least exceed the average kineticenergy during storage, and preferably exceed it by an order of magnitudeor more. Such considerable field strength reduces the time of flightspread caused by the initial energies.

The optimum duration of trapping of ions within the linear trap 30 maybe determined using a pre-scan after a short storage duration. Thesecondary electron multiplier 190 allows detection. Where the SEM 190 islocated radially of the linear trap 30, mass-selective instability or aresonance excitation scan in the linear trap 30 may be used. It ispreferable, however, to use an axial SEM 190, after the orbitrap 130, sothat ions are injected in the same way as for the analysis in theorbitrap 130. It will be understood by those skilled in the art that anyother known way of ion detection could be used instead of SEM 190, suchas a collector with a charge-sensitive amplifier, a photo-multiplier,semiconductor detectors and so forth.

During detection of ions in the orbitrap 130, an appropriate voltage,which may be time-dependent, may be supplied to a field compensator 200adjacent the entrance to the orbitrap 130. This field compensatorensures minimum field perturbation within the volume occupied by the iontrajectories. During ion injection, this field compensator 200 acts alsoas a deflector to improve trapping efficiency within the orbitrap 130.

Although a preferred embodiment has been described, it will beunderstood that various modifications are contemplated. For example,although a linear ion trap 30 has been described for storage of thesample ions from the ion source 12, it is to be understood that aquadrupole ion trap could equally be employed for ion storage, coolingand so forth. Quadrupole ion traps are known as such and one example isshown in EP-A-0,113,207.

Furthermore, a second form of orbitrap 130, seen in front view in FIG.5, may be employed instead of the one shown in FIG. 3. Here, instead ofsplit outer electrode parts, the outer electrode is not split. Instead,the central electrode 140′ is axially segmented with a centre part 220connected to a pre-amplifier 210. In this arrangement, pulses of imagecurrent from passing ion packets of different m/z are detected by thiscentral, disc-shaped electrode part 220, amplified by the pre-amplifier210, and then processed to yield a time of flight spectrum. Adeconvolution method such as is described by M. May et al, in AnalyticalChemistry (1992) vol. 64, pages 1601-1605 could be used, or any othersignal reconstruction method. Two or more detection electrodes couldalso be used. The pre-amplifier could also be a differentialpre-amplifier with a second input connected to another detectionelectrode or simply open-ended to improve common mode rejection. Forthis type of detection, the best results are achieved when the durationdT (m/z) of each ion packet of a particular m/z is not only smaller thanthe period of axial oscillations in the orbitrap 130, but also does notexceed the time of flight along each of the detection electrodes.

Furthermore, a mass selective instability scan, such as is described inthe above-referenced U.S. Pat. No. 5,886,346 could also be used. In thiscase, ion injection could be performed also along the central plane ofthe trap.

FIG. 6 shows another mass spectrometer 10′ which embodies the presentinvention and currently represents the preferred implementation.Features common to FIGS. 1 and 6 are labelled with like referencenumerals.

As with the arrangement of FIG. 1, the mass spectrometer 10′ comprisesan electrospray ion source 12 which provides nebulized ions into an ionsource block 16 through an entrance cone 14. The ions exit the block 16via an exit cone 18 and pass into an ion cooler 20 at around 10⁻² mbar.The ions then arrive in a quadrupole mass filter 24 via an aperture 22,and a range of m/z of the incident ions is selected as describedpreviously.

Ions exiting the mass filter 24 enter an ion trap 300 in a firstdirection generally parallel with a ‘y’ axis (see FIG. 6). The ions arehowever ejected from the ion trap 300 in a second direction generallyorthogonal to their entrance direction, that is, in an ‘x’ direction asindicated in FIG. 6. As previously, the ions are focussed in time offlight downstream of the linear trap. Orthogonal ejection of trappedions additionally allows a much higher space charge capacity than isprovided by the arrangement of FIG. 1, and also provides better ion beamparameters.

Although, to achieve orthogonal ejection, a quadrupole assembly such asis shown in FIG. 1 could be used, this requires the RF voltages appliedthereto to be switched off instantaneously. This requires very complexassociated electronics, however, so, in preference, the ion trap 300 iselongate in the ‘y’ direction but is curved in the x-y plane (such thatthe longitudinal axis is likewise curved in the x-y plane). Curvature ofthe ion trap 300 improves geometrical focussing. By way ofdistinguishing from the truly linear trap 130 of FIG. 1, the ion trap300 will henceforth be termed a ‘curved trap’.

The curved trap 300 includes lenses 310 extending from the exit of thetrap which together convert the wide angle incident beam into a narrowbeam.

The narrow ion beam exiting the curved trap 300 passes through a beamdeflector 320 and into an electrostatic trap 130. The beam deflector maytake one of a variety of forms. A plate in front of the focal point ofthe focussing lenses could be used to block both ions and gas along theline of sight towards the entrance to the electrostatic trap 130 (forexample, around ±5° of arc). Gas arriving from larger angles will not bealong the line of sight whereas corresponding ions can be diverted intothe electrostatic trap entrance by lenses. The problem with thisapproach is that it requires the blocking plate to be located in afield-free region which can be difficult to arrange. As an alternative,a toroidal deflector can be employed to achieve the required beamdeflection although this introduces extra complexity.

The preferred beam deflector 320 is shown in FIG. 6 and contains aright-angled bend which prevents gas carryover along the line of sightbetween the curved trap 300 and the electrostatic trap 130. An‘S’-shaped beam deflector could of course be employed instead (as isshown in FIG. 1). If the electrostatic trap is arranged with itsentrance parallel to the direction of exit of ions from the curved trap300. As with the arrangement of FIG. 1, the ions are usually focussed intime of flight onto the electrostatic trap entrance which then detectsin the manner described previously. However, a different focal point canbe chosen, as for example when operating in surface-induced dissociationmode (see FIG. 8 below).

Referring now to FIG. 7, a sectional view of the curved trap 300 of FIG.6 is shown. The curved trap 300 is preferably RF only and comprises anouter electrode plate 330, at a D.C. voltage preferably near ground,along with an inner electrode plate 340 at the same D.C. voltage.Sandwiched between the inner and outer electrode plates 330, 340 areupper and lower centre plate pairs 360 a, 360 b, to which is applied anRF voltage having a phase indicated on the drawings as RF−. The upperand lower centre plate pairs 360 a, 360 b in turn sandwich an axis platepair 350 to which is applied an RF voltage (labelled RF+ in FIG. 7)which is in antiphase to the voltage applied to the upper and lowercentre plate pairs 360 a, 360 b.

The geometry of the curved trap 300 is chosen in such a way that theminimum of the effective potential (that is, the minimum of the RFfield) is located exactly in the middle of the trap. This is the axis ofsymmetry in the plane XZ and is labelled point Q in FIG. 7.

The electrodes 330, 340, 350, 360 a and 360 b are curved in the XYplane. A DC offset, which is the same as that applied to the inner andouter electrode plates 330, 340, is applied to the upper, lower and axisplates. This causes all masses to be stored in the curved trap 300 andcooled in collisions with gas at 0.1-1 mtorr. At the end of the storage,a positive pulse is applied to the outer electrode plate 330, and anegative pulse of the same amplitude is applied to the inner electrodeplate 340 (for positive ions). Ions are extracted by the resultingelectric field through a trap exit 370. The RF voltages do not need tobe removed as they have little effect on the beam parameters due totheir symmetry. In addition, the RF field strength near the X axis isrelatively weak. However, it is preferable to time the pulses applied tothe inner and outer electrode plates 330, 340 so that they are appliedin synchronism with the phase of the RF voltages applied to the upper,lower and axis plates 350, 360 a, 360 b.

After sufficient ions have been stored in the trap, they are ejectedtowards the centre of curvature of the curved trap 300 (see below), andalso focussed, both geometrically and in time-of-flight, into a narrowbeam that is then introduced into the electrostatic trap 130.

Still referring to FIG. 7, a liner 380 may be provided between the trapexit 370 and the lenses 310. The liner 380 is located in a substantiallyfield-free region of the curved trap. Another pulse, equal to therequired ion energy on the entrance to the electrostatic trap 130, maybe applied to the liner 380 at the same time as the pulses are appliedto the inner and outer electrode plates 330, 340. The pulse is appliedto the liner so as to create an “energy lift”, that is, to produce ahigh ion energy in conditions where both the DC offset applied to thecurved trap 300 and the potential applied to the outer electrode parts160, 170 of the orbitrap 130 (FIG. 3) are maintained near groundvoltage. If the curved trap 300 floats at the acceleration voltage thenno energy lift will be required. Nevertheless, it is important that anyion source should have the capability to float as well.

The length of the liner 380 is defined by the required relative massrange: the ratio of the maximum to minimum masses is given bym_(max)/m_(min)=(L₁/L₂)², where L₁ is the effective distance from theaxis to the exit from liner and L₂ is the effective distance to theentrance to the liner. The duration of the pulse applied to the liner380 is determined by calculating the time-of-flight, to the liner exit,for the lightest mass of interest, so that this mass emerges from theliner 380 at its full energy whilst the voltage on the liner 380 isalready back to its normal value (near ground). The duration of thepulses applied to the inner and outer electrode plates 330, 340 shouldbe longer than this.

It is to be understood that the liner 380 in the curved trap 300 ofFIGS. 6 and 7 is equally suitable for the linear trap 30 of FIG. 1.

The foregoing description of some preferred embodiments has alsoexplained the principles involved with reference to sample ions that arederived directly from an atmospheric pressure ionization source or thelike, and are simply stored as such in the ion trap. However, structuralanalysis of sample ions may also be carried out using TOF focussing andany of the ms/ms modes available. Three modes in particular arecontemplated.

In collision-induced dissociation (CID), precursor ions may be selectedeither by the quadrupole mass filter 24 (FIG. 1) or inside the ion trap30. Ejection of unwanted ions in each of these cases may be performed,for example, by resonant excitation between the opposite rods of eachdevice or by a mass selective instability scan (see, for example, theabove-referenced U.S. Pat. No. 5,886,346). This may be achieved by DCbiassing one set of rods relative to the other, for example.Fragmentation may be caused by collisions with collision gas at anelevated pressure in a dedicated RF-only multipole or, preferably, inthe linear ion trap 30. The resulting fragments are stored andcollisionally cooled in the ion trap 30 and then injected into theorbitrap 130 in the same way as described previously. CID in collisionalmultipoles is a technique which is well known as such to those skilledin the art and the technique of selecting only the required m/z islikewise a known part of tandem mass spectrometry. Further descriptionof these techniques may be found in “Protein Sequencing andIdentification Using Tandem Mass Spectrometry”, by M. Kinter, N. E.Sherman, John Wiley and Sons, 2000, and in “Mass Spectrometry/MassSpectrometry: Techniques and Applications of Tandem Mass Spectrometry”,by K. L. Busch, G. L. Glish, and S. A. McLuckey, John Wiley and Sons,1989.

In surface-induced dissociation (SID), precursor ion selection istypically carried out in the same way as in CID. Precursor ions arestored in the linear trap 30 and then pulsed towards the orbitrap 130.However, in this case the TOF focus is shifted behind the plane of theentrance of the orbitrap 130, to a separate plane where a collisionsurface is located. This is shown schematically in FIG. 8, whichillustrates in side view the orbitrap 130 along with the electrode 140,the compensator 200 and the secondary electron multiplier 190. Thecollision surface 250 is located downstream of the secondary electronmultiplier 190 and may be formed from a metal or may instead be metal-or polymer-coated. A fluorocarbon or hydrocarbon self-assembledmonolayer plate such as is disclosed in Science, Vol. 275, pages1447-1450, by S. A. Miller, H. Luo, S. J. Pachuta and R. G. Cooks,(1997) could for example be used.

In this case, precursor ions pass tangentially through the orbitrap 130,which has electric fields that are low enough to prevent ion losses, andout past the compensator 200 which in this part of the process isswitched off to allow passage of the precursor ions. These precursorions then decelerate in front of the collision surface 250 in adeceleration gap created by a grid 255 and collide with the collisionsurface 250 at a collision energy determined by a voltage differencebetween the collision surface and the offset applied to the exit segment50 of the linear trap. Collision results in the formation of fragmentions at low energies (several electronvolts) which are accelerated bythe same electric field towards the orbitrap 130. Due to the TOFfocussing of these precursor ions and the instantaneous nature of SID,fragment ions separate (at least partially) on their way to the orbitrap130 according to their mass/charge ratio and ions of each mass/chargeratio enter the orbitrap 130 as a short packet. The compensator 200 isswitched on during this part of the process so that the fragment ionsare then captured by the orbitrap. This allows ions to be captured inthe same way as in the MS-only mode described in connection with FIGS. 1to 5 above. If low resolution of parent selection is sufficient for agiven application, then an ion gate (not shown) may be installed betweenthe orbitrap 130 and the collision surface 250 to provide an alternativeway of selecting precursor ions. It is in fact possible to use thedeflection electrode 200 of the orbitrap 130 as an ion gate.

Finally, metastable dissociation (MSD) mode may be employed with thearrangement and principles outlined previously. Precursor ions mayeither be selected as described above in connection with the CID and SIDmodes, or may instead be injected into the orbitrap 130 without priorselection. The only difference from the MS-only mode described inconnection with FIGS. 1 to 5 is the activation of ions in the ion trap30. Activation may be achieved by pulsed ion extraction in the presenceof gas at an elevated pressure (either static or pulsed), wherein theincrease of ion internal energy is controlled by the gas pressure, bypulsed electromagnetic radiation (e.g. infrared radiation which can beused to excite ions inside the ion trap 30, or by resonant or broadbanddipolar excitation using at least two pairs of quadrupole rod electrodesin the ion trap 30. In that case, the increase of internal energy iscontrolled by the amplitude of the excitation signal and the gaspressure.

Although pulsed ion extraction with a high pressure gas is preferabledue to its simplicity, each of the foregoing results in the excitation(“heating”) of ions and the consequent formation of metastable ions witha controllable decay constant. The magnitude of the decay constant canbe controlled by variation of the intensity of excitation. Beforefragmentation becomes noticeable, ions may be injected into the orbitrap130 and precursor ion selection may even be achieved. On the other hand,excessively long decay times lead to a decrease in the speed ofanalysis. Therefore, optimum decay times range from several millisecondsto tens of milliseconds.

Precursor ion selection is achieved by applying a radio frequencyvoltage in resonance with the axial oscillation of precursor ions at acorrect phase. A waveform generator (not shown), under the control ofthe data processing system referred to in connection with FIG. 1,supplies this RF voltage either to the central electrode 140 (parametricresonance de-excitation at doubled ion frequency, set out in AnalyticalChemistry Volume 72, No. 6, p. 1156-1162, by Makarov, and in theabove-referenced U.S. Pat. No. 5,886,346), or between the two outerelectrode parts 160, 170 (resonance de-excitation at ion frequency) ofthe orbitrap 130. Application of an RF voltage decreases the amplitudeof axial oscillation of ions so that only precursor ions are broughtinto the plane of symmetry of the orbitrap 130. Precursor ions are leftin this state long enough to allow metastable decay to occur. Theremaining precursor ions are then excited, together with their fragmentions, by a broadband excitation. Typically, a radio frequency voltage isapplied to the two outer electrode parts 160, 170 by the waveformgenerator. Coherent oscillations of ions of each mass/charge ratio aredetected by detecting an image detection current via the differentialamplifier 180 in the same way as described for MS-only mode. Metastabledecay of ions other than precursor ions also results in the formation offragment ions. However, these are uniformly spaced along the orbitrap130 and thus do not move coherently. No image current is produced fordetection in that case. Alternatively, unwanted precursor or fragmentions may be removed by an additional broadband excitation.

1. (canceled)
 2. A mass spectrometer comprising: an ion source forsupplying sample ions for analysis; an ion trap having an entrancethrough which the sample ions are received, and an exit through whichthe sample ions are released; a voltage source being configured to applya first set of trapping voltages to the ion trap to confine the sampleions within the ion trap, the first set of trapping voltages beingselected to cause a center of an ion cloud defined by the sample ionscontained within the ion trap to be axially shifted toward the exit; thevoltage source being further configured to apply a release voltage tothe ion trap to release the sample ions from the ion trap through theexit.
 3. The mass spectrometer of claim 2, wherein the ion trap is anaxially segmented linear ion trap having a first segment locatedrelatively closer to the exit than a second segment, and the voltagesource is configured to apply different voltages to the first and secondsegments.
 4. The mass spectrometer of claim 2, wherein the voltagesource is configured to apply an initial set of trapping voltages to theion trap, prior to applying the first set of trapping voltages, suchthat the center of the ion cloud is positioned relatively more remotefrom the exit when the initial set of voltages is applied and is movedcloser to the exit when the first set of voltages are applied.
 5. Themass spectrometer of claim 2, wherein the ion trap contains a collisiongas to damp the oscillatory movement of the sample ions.
 6. The massspectrometer of claim 2, further comprising an electrostatic trappositioned and configured to receive and trap sample ions released fromthe storage device.
 7. The mass spectrometer of claim 2, wherein uponapplication of the release voltage, the electric field experienced bysample ions in the ion cloud is substantially uniform.
 8. The massspectrometer of claim 2, wherein the release voltage is selected suchthat sample ions released from the storage device arrive at theelectrostatic trap as a convolution of bunched time of flightdistributions for each mass-to-charge ratio.
 9. An ion trap for trappingsample ions in a mass spectrometer, comprising: an entrance throughwhich the sample ions are received, and an exit through which the sampleions are released; and a voltage source being configured to apply afirst set of trapping voltages to the ion trap to confine the sampleions within the ion trap, the first set of trapping voltages beingselected to cause a center of an ion cloud defined by the sample ionscontained within the ion trap to be axially shifted toward the exit; thevoltage source being further configured to apply a release voltage tothe ion trap to release the sample ions from the ion trap through theexit.
 10. A method for injecting sample ions into an electrostatic trap,comprising the steps of: receiving the sample ions through an entranceof an ion trap; applying a first set of trapping voltages to the iontrap to confine the sample ions within the ion trap, the first set oftrapping voltages being selected to cause a center of an ion clouddefined by the sample ions contained within the ion trap to be shiftedtoward an exit of the ion trap; applying a release voltage to the iontrap to release the sample ions from the ion trap through the exit; andreceiving the released sample ions through an entrance of theelectrostatic trap.
 11. The method of claim 10, wherein the step ofapplying a first set of trapping voltages includes applying differentvoltages to first and second segments of an axially segmented lineartrap.
 12. The method of claim 10, further comprising the step of: priorto applying the first set of trapping voltages, applying an initial setof trapping voltages such that the center of the ion cloud is positionedrelatively more remote from the exit when the initial set of voltages isapplied and is moved closer to the exit when the first set of voltagesare applied.
 13. The method of claim 10, further comprising the step ofproviding a collision gas in the ion storage device to damp theoscillatory motion of the ion trap.
 14. The method of claim 10, whereinupon application of the release voltage, the electric field experiencedby sample ions in the ion cloud is substantially uniform.
 15. The methodof claim 10, wherein the release voltage is selected such that sampleions released from the storage device arrive at the electrostatic trapas a convolution of bunched time of flight distributions for eachmass-to-charge ratio.